<h2>DESCRIPTION</h2> 

t.select performs selection of maps that are registered in space time datasets using temporal algebra.

<h3>PROGRAM USE</h3> 
The module expects an <b>expression</b> as input parameter in the following form: <br>
<br>
<b> "result = expression" </b>
<br>
<br>

The statement structure is similar to r.mapcalc, see <a href="r.mapcalc.html">r.mapcalc</a>.
Where <b>result</b> represents the name of a space time dataset (STDS)that will 
contain the result of the calculation that is given as <b>expression</b> 
on the right side of the equality sign. 
These expression can be any valid or nested combination of temporal 
operations and functions that are provided by the temporal algebra.  <br>
The temporal algebra works with space time datasets of any type (STRDS, STR3DS and STVDS). 
The algebra provides methods for map selection from STDS based on their temporal relations. 
It is also possible to temporally shift maps, to create temporal buffer and to snap time 
instances to create a valid temporal topology. Furthermore expressions can be nested and 
evaluated in conditional statements (if, else statements). Within if-statements the algebra 
provides temporal variables like start time, end time, day of year, time differences or 
number of maps per time interval to build up conditions. These operations can be assigned 
to space time datasets or to the results of operations between space time datasets. 
<br>
<br>
The type of the input space time datasets must be defined with the input 
parameter <b>type</b>. Possible options are STRDS, STVDS or STR3DS. 
The default is set to space time raster datasets (STRDS).
<br>
<br>
As default, topological relationships between space time datasets will be 
evaluated only temporal. Use the <b>s</b> flag to activate the 
additionally spatial topology evaluation.
<br>
<br>
The expression option must be passed as <b>quoted</b> 
expression, for example: <br>
<div class="code"><pre>t.select expression="C = A : B"</pre></div> 
Where <b>C</b> is the new space time raster dataset that will contain maps
from <b>A</b> that are selected by equal temporal relationships 
to the existing dataset <b>B</b> in this case. <br>
<br>
<h2>TEMPORAL ALGEBRA</h2>

The temporal algebra provides a wide range of temporal operators and 
functions that will be presented in the following section. <br>
<br>

<h3>TEMPORAL RELATIONS</h3>

Several temporal topology relations between space 
time datasets are supported: <br>
<div class="code"><pre>
equals            A ------
                  B ------

during            A  ---- 
                  B ------

contains          A ------
                  B  ---- 

starts            A ----
                  B ------

started           A ------
                  B ----

finishs           A   ---- 
                  B ------

finished          A ------
                  B   ----

precedes          A ----
                  B     ----

follows           A     ----
                  B ----

overlapped        A   ------
                  B ------

overlaps          A ------
                  B   ------

over              booth overlaps and overlapped

</pre></div>
The relations must be read as: A is related to B, like - A equals B - A is 
during B - A contains B <br>
<br>
Topological relations must be specified in {} parentheses. <br>

<h3>TEMPORAL SELECTION</h3>

The temporal selection simply selects parts of a space time dataset without 
processing raster or vector data.

The algebra provides a selection operator <b>:</b> that selects parts 
of a space time dataset that are temporally equal to parts of a second one 
by default. The following expression
<div class="code"><pre>
C = A : B
</pre></div>
means: Select all parts of space time dataset A that are equal to B and store 
it in space time dataset C. The parts are in this case time stamped maps. <br>
<br>
In addition the inverse selection operator <b>!:</b> is defined as the complement of 
the selection operator, hence the following expression
<div class="code"><pre>
C = A !: B
</pre></div>
means: select all parts of space time time dataset A that are not equal to B 
and store it in space time dataset (STDS) C. <br>
<br>
To select parts of a STDS by different topological relations to other STDS, 
the temporal topology selection operator can be used. The operator consists of 
topological relations, that must be separated by the logical OR operator 
<b>||</b> and the temporal selection operator. Both parts are separated by 
comma and surrounded by curly braces: 
{"topological relations", "temporal selection operator"}  <br>
<br>
Examples:
<div class="code"><pre>
C = A {equals,:} B
C = A {equals,!:} B
</pre></div>
We can now define arbitrary topological relations using logical OR operator 
to connect them:
<div class="code"><pre>
C = A {equals||during||overlaps,:} B
</pre></div>
Select all parts of A that are equal to B, during B or overlaps B. <br>
<br>
The selection operator is implicitly contained in the temporal topology 
selection operator, so that the following statements are exactly the same:
<div class="code"><pre>
C = A : B
C = A {:} B
C = A {equal,:} B
</pre></div>

Same for the complementary selection:
<div class="code"><pre>
C = A !: B
C = A {!:} B
C = A {equal,!:} B
</pre></div>

<h3>CONDITIONAL STATEMENTS</h3>

Selection operations can be evaluated within conditional statements.
<br>
<div class="code"><pre>
Note A and B can either be space time datasets or expressions.

if statement                         decision option                        temporal relations
  if(if, then, else)
  if(conditions, A)                    A if conditions are True;              temporal topological relation between if and then is equal.
  if(conditions, A, B)                 A if conditions are True, B otherwise; temporal topological relation between if, then and else is equal.
  if(topologies, conditions, A)        A if conditions are True;              temporal topological relation between if and then is explicit specified by topologies.
  if(topologies, conditions, A, B)     A if conditions are True, B otherwise; temporal topological relation between if, then and else is explicit specified by topologies.
</pre></div>
The conditions are comparison expressions that are used to evaluate 
space time datasets. Specific values of temporal variables are 
compared by logical operators and evaluated for each map of the STDS.
<br>
<br>
The supported logical operators:
<div class="code"><pre>
Symbol  description

  ==    equal
  !=    not equal
  >     greater than
  >=    greater than or equal
  <     less than
  <=    less than or equal
  &&    and
  ||    or
</pre></div>

Temporal functions: <br>
<div class="code"><pre>

td(A)                   Returns a list of time intervals of STDS A

start_time()            Start time as HH::MM:SS
start_date()            Start date as yyyy-mm-DD
start_datetime()        Start datetime as yyyy-mm-DD HH:MM:SS
end_time()              End time as HH:MM:SS
end_date()              End date as yyyy-mm-DD
end_datetime()          End datetime as  yyyy-mm-DD HH:MM

start_doy()             Day of year (doy) from the start time [1 - 366]
start_dow()             Day of week (dow) from the start time [1 - 7], the start of the week is Monday == 1
start_year()            The year of the start time [0 - 9999]
start_month()           The month of the start time [1 - 12]
start_week()            Week of year of the start time [1 - 54]
start_day()             Day of month from the start time [1 - 31]
start_hour()            The hour of the start time [0 - 23]
start_minute()          The minute of the start time [0 - 59]
start_second()          The second of the start time [0 - 59]
end_doy()               Day of year (doy) from the end time [1 - 366]
end_dow()               Day of week (dow) from the end time [1 - 7], the start of the week is Monday == 1
end_year()              The year of the end time [0 - 9999]
end_month()             The month of the end time [1 - 12]
end_week()              Week of year of the end time [1 - 54]
end_day()               Day of month from the start time [1 - 31]
end_hour()              The hour of the end time [0 - 23]
end_minute()            The minute of the end time [0 - 59]
end_second()            The second of the end time [0 - 59]            
</pre></div>

Additionally the number of maps in intervals can be computed and 
used in conditional statements. <br>
The operator to count the number of maps 
is the hash <b>#</b>. 
<div class="code"><pre>
A{contains,#}B
</pre></div>
This expression computes the number of maps from space 
time dataset B which are during the time intervals of maps from 
space time dataset A.<br>
A list of integers (scalars) corresponding to the maps of A 
that contain maps from B will be returned. <br>
<br>

Furthermore the temporal algebra allows temporal buffering, shifting 
and snapping with the functions buff_t(), tshift() and tsnap()
respectively.
<div class="code"><pre>
buff_t(A, size)         Buffer STDS A with granule ("1 month" or 5)
tshift(A, size)         Shift STDS A with granule ("1 month" or 5)
tsnap(A)                Snap time instances and intervals of STDS A
</pre></div>

<h3>Examples: </h3>

Select all maps from space time dataset A which have equal time stamps 
with space time dataset B and C and are ealier that Jan. 1. 2005 and 
store them in space time dataset D.
<div class="code"><pre>
D = if(start_date() < "2005-01-01", A : B : C)
</pre></div>

Select all maps from space time dataset A which contains more than three
maps of space time dataset B, else select maps from C with time
stamps that are not equal to A and store them in space time dataset D.
<div class="code"><pre>
D = if(A {contain, #} B > 3, A {contain, :} B, C)
</pre></div>

Select all maps from space time dataset B which are during the temporal 
buffered space time dataset A with a map interval of three days, else
select maps from C and store them in space time dataset D.
<div class="code"><pre>
D = if(contain, td(buff_t(A, "1 days")) == 3, B, C)
</pre></div>

<h2>REFERENCES</h2>

<tt><a href="http://www.dabeaz.com/ply/">PLY(Python-Lex-Yacc)</a></tt>

<h2>SEE ALSO</h2>

<em>
<a href="v.mapcalc.html">v.mapcalc</a>
</em>


<h2>AUTHOR</h2>

Thomas Leppelt, Soeren Gebbert, Thuenen Institut, Germany <br>

<p><i>Last changed: $Date: 2013-07-24 17:35:20 +0100 (Thu, 24 Jul 2013) $</i>

